Coefficient of thermal expansion (CTE) matched transistor outline (TO) header

ABSTRACT

A transistor outline (TO) package may include a TO cap. The TO package may include a TO header. The TO header may include a header stem of a first material and a first coefficient of thermal expansion (CTE) value. The TO header may include a header base of a second material and a second CTE value. The first material and the second material may be different. The first CTE value and the second CTE value may be different. The first CTE value and the second CTE value may be within a threshold percentage of each other.

RELATED APPLICATION(S)

This application claims priority under 35 U.S.C. § 119 to U.S.Provisional Patent Application No. 62/383,244, filed on Sep. 2, 2016,the content of which is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The present disclosure relates to transistor outline (TO) packages. Moreparticularly, the present disclosure relates to a coefficient of thermalexpansion (CTE) matched TO header for a TO package.

BACKGROUND

Transistor outline (TO) packages include a TO header and a TO cap.Optical components, such as a laser diode, may be mechanically coupledto the TO header. For example, a laser diode may be included in a chipon sub-mount assembly that is attached to the TO header. The TO headermay include a header base and a header stem. For example, the chip onsub-mount assembly, which includes the laser diode, is attached to theheader stem of the TO header, and the header stem is attached to theheader base of the TO header. The TO cap may support a window or lens topermit the laser diode mounted to the TO header to be optically coupledto another optical component disposed external to the TO package or anoptical fiber. The TO cap may be affixed to a header base of the TOheader using a welding procedure, thereby providing a hermetic seal foroptical components mounted to the header stem and aligning a lens orwindow to a centerline of the TO package and to the optical componentsmounted to the header stem.

Copper may be selected for the header stem to provide a low thermalresistance for the TO package, thereby permitting heat to be removedfrom the TO package during operation of the optical components, such asthe laser diode. Steel may be selected for the header base to permit theTO cap to be welded to the header base to form the hermetic seal.However, a coefficient of thermal expansion (CTE) mismatch between thesteel header base and the copper header stem results in thermal stress.The thermal stress distorts the copper header stem from a centerline ofthe TO package at a change of temperature of the TO package from anambient temperature to an operating temperature. Accordingly, it may bedesirable for a thermally dissipating TO package to include a CTEmatched header stem, such as a copper-tungsten header stem, and headerbase to maintain an alignment and optical coupling of optical componentsinside the TO package to corresponding optical components external tothe TO package at an operating temperature of the optical componentsinside the TO package.

SUMMARY

According to some possible implementations, a transistor outline (TO)header may include a copper-tungsten header stem with a firstcoefficient of thermal expansion (CTE) value. The TO header may includea header base with a second CTE value that is different than the firstCTE value. The header base may be a different material from thecopper-tungsten header stem.

According to some possible implementations, an optical module mayinclude a laser diode. The optical module may include a transistoroutline (TO) cap. The optical module may include a TO header. The TOheader may include a header base to attach to the TO cap. The TO headermay include a copper-tungsten header stem to attach to the header baseand to mount the laser diode.

According to some possible implementations, a transistor outline (TO)package may include a TO cap. The TO package may include a TO header.The TO header may include a header stem of a first material and a firstcoefficient of thermal expansion (CTE) value. The TO header may includea header base of a second material and a second CTE value. The firstmaterial and the second material may be different. The first CTE valueand the second CTE value may be different. The first CTE value and thesecond CTE value may be within a threshold percentage of each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrams of an overview of an example implementationdescribed herein;

FIGS. 2A and 2B are diagrams of an example implementation of a CTEmatched TO package;

FIGS. 3A and 3B are diagrams of a finite element analysis relating tothe example implementations described herein; and

FIG. 4 is a diagram of a temperature-induced coupling change relating tothe example implementations described herein.

DETAILED DESCRIPTION

The following detailed description of example implementations refers tothe accompanying drawings. The same reference numbers in differentdrawings may identify the same or similar elements.

A laser diode provides laser light to an optical communicationstransmitter or an optical communications transceiver to enable opticalcommunication. The laser diode may be included in a chip on sub-mountassembly. The chip on sub-mount assembly may affix the laser diode to asub-mount substrate or incorporate the laser diode into the sub-mountsubstrate, and may provide a heat spreading functionality for the laserdiode. The chip on sub-mount assembly is attached to an optical module,such as a transistor outline (TO) package.

The TO package may include a TO header and a TO cap. The TO header mayinclude a header base and a header stem. The chip on sub-mount assemblyis attached to the header stem to align the laser diode with acenterline of the TO package, and optically couple the laser diode withone or more optical components of the optical communications transmitteror optical communications transceiver that are external to the TOpackage. Thus, it may be advantageous to ensure that the laser dioderemains in alignment with the centerline of the TO package and isprovided a threshold amount of heat dissipation by the TO package.

Steel may be selected for the header base to ensure that the TO cap canbe welded to the header base to provide a hermetic seal of the laserdiode and to align a window or lens of the TO cap with the laser diode.Copper may be selected for the header stem to ensure a low thermalresistance, thereby providing the threshold amount of heat dissipation.However, the copper header stem is relatively soft, and distorts awayfrom a first position at an ambient temperature to a second position atan operating temperature of the laser diode. Moreover, a coefficient ofthermal expansion (CTE) mismatch between the copper header stem and thesteel header base forestalls compensation of the distortion via thermalexpansion or thermal contraction, thus it may be advantageous to CTEmatch the header stem and the header base.

Implementations, described herein, may provide a copper-tungsten basedheader stem for a TO package, thereby reducing a temperature-inducedcoupling change without negatively impacting electrical performance ofthe TO package. Based on reducing the temperature-induced couplingchange, the copper-tungsten based header stem improves couplingefficiency of the laser diode for the optical communications transmitteror the optical communications transceiver, thereby improving opticalcommunications system performance.

FIGS. 1A and 1B are an overview of an example implementation 100. FIG.1A shows an overview of a pump laser assembly that includes a TOpackage. As shown in FIG. 1A, example implementation 100 includes a pumplaser assembly 102, which includes a fiber tail assembly (FTA) 104, asleeve 106, a housing 108, a header base 110 of the TO package, a lens112, and a header stem 114 of the TO package.

Pump laser assembly 102 includes a laser diode that emits a laser beamfor an optical communications transmitter or an optical communicationstransceiver of an optical communications system. In someimplementations, pump laser assembly 102 may operate at a particulartemperature range. For example, pump laser assembly 102 may operate atan operating temperature of between approximately 0 degrees Celsius (C.)and approximately 75 degrees C. Pump laser assembly 102 includes a setof components, such as FTA 104, sleeve 106, and/or housing 108, tooptically couple the laser diode with an optical fiber, another opticalcomponent, and/or the like. Pump laser assembly 102 mounts lens 112,which focuses a laser beam emitted from the laser diode at lightemitting point 116 toward light receiving point 118, as shown in detailwith regard to call-out diagram 120.

Header base 110 is a TO header base of a TO header. Header base 110 maybe attached to header stem 114. Header stem 114 is a TO header stem ofthe TO header. The laser diode is mounted onto header stem 114. Headerbase 110 may attach to a TO cap, which may provide a hermetic seal ofthe laser diode mounted onto header stem 114. Header base 110 and headerstem 114 are positioned to align the laser diode to centerline 122 ofpump laser assembly 102 to ensure that the laser diode is opticallycoupled to an optical fiber, another optical component, and/or the like.

Pump laser assembly 102 is associated with a particular couplingefficiency of the laser diode, which may be temperature dependent. Atemperature-induced coupling change (TCC) relating to the couplingefficiency is determined as:TCC_(Tcase)=(P _(Tcase) /P _(ref))−1;

where TCC_(Tcase) is a temperature-induced coupling change at anoperating temperature of pump laser assembly 102, P_(Tcase) is an outputpower of pump laser assembly 102 at the operating temperature, andP_(ref) is an output power of pump laser assembly 102 at a referencetemperature or ambient temperature (e.g., 25 degrees C.). Based on atemperature change, a position of light emitting point 116 (e.g., aposition of a laser diode mounted header stem 114) is altered as aresult of thermal distortion.

In this case, using a coefficient of thermal expansion (CTE) matcheddesign results in a reduced thermal distortion of the laser diode fromcenterline 122 at an operating temperature of pump laser assembly 102,thereby causing a reduced TCC for pump laser assembly 102 relative toanother design. For example, header base 110 may be manufactured fromsteel to enable welding of a TO cap to header base 110 and header stem114 may be manufactured from a copper (Cu)-tungsten (W) based material,such as a CuW50 material, a CuW60 material, a CuW70 material, and/or thelike. In this case, the CTE match may be within a threshold percentage,such as within approximately 25%, within approximately 20%, withinapproximately 15%, within approximately 10%, and/or the like. Further tothe example, the copper-tungsten based material ensures a thresholdamount of heat dissipation for the laser diode, thereby reducing alikelihood of the laser diode overheating relative to another material.

As shown in FIG. 1B, during operation of pump laser assembly 102, alaser beam is directed, via lens 112, from laser diode (LD) 126 at lightemitting point 116 toward light receiving point 118 and toward FTA 104.Laser diode 126 may be disposed away from lens 112 by a distance alongan optical axis of, for example, approximately 1.00 millimeters (mm).Lens 112 may be associated with a length along the optical axis of, forexample, approximately 1.00 mm. Lens 112 may be disposed away from lightreceiving point 118 along the optical axis by, for example,approximately 2.18 mm.

As further shown in FIG. 1B, chart 140 shows an example of opticalperformance of another pump laser based on an offset of a laser diode ofthe other pump laser (Chip XY). For example, with regard to thermaldistortion causing displacement of the laser diode of the other pumplaser in the X axis, a distortion of 0.0005 mm results in a 5% reductionin coupling efficiency (CE drop). Similarly, with regard to thermaldistortion causing displacement of the laser diode of the other pumplaser in the Y axis, a distortion of 0.0005 mm results in a 10%reduction in coupling efficiency. Accordingly, reduction in thermaldistortion in the X axis and more particularly in the Y axis of pumplaser assembly 102, relative to the other pump laser, results inimproved coupling efficiency (i.e., a reduced coupling efficiencyreduction) for pump laser assembly 102.

In another example, a similar CTE matched TO package may improveperformance of a non-fiber coupled laser package, such as a transmitoptical sub-assembly (TOSA) package and/or the like.

As indicated above, FIGS. 1A and 1B are provided merely as an example.Other examples are possible and may differ from what was described withregard to FIGS. 1A and 1B.

FIGS. 2A and 2B are diagrams of an example implementation 200. FIGS. 2Aand 2B show an example of a CTE matched TO package.

As shown in FIG. 2A, a TO package includes a TO header 202 and a TO cap204. TO cap 204 may be attached to TO header 202 to provide a hermeticseal for a laser diode mounted to TO header 202. TO header 202 and TOcap 204 may be manufactured using a copper-tungsten based material, suchas CuW50 or the like, a steel based material, a combination of acopper-tungsten based material and a steel based material, and/or thelike. In another example, TO header 202 and TO cap 204 may bemanufactured using a copper-molybdenum based material, such as a CuMo60material. In some implementations, the material from which TO header 202and TO cap 204 are manufactured may be selected to permit TO cap 204 tobe attached to TO header 202, such as by a welding process. TO cap 204may include an opening, which may mount a lens, a glass, a filter, orthe like to permit a laser diode mounted to TO header 202 to opticallycouple to another optical component, an optical fiber, and/or the like.

As shown in FIG. 2B, TO header 202 includes a header base 210, a headerstem 212, and a chip on sub-mount assembly 214. Header base 210 mayreceive TO cap 204, when TO cap 204 is to be attached to TO header 202.For example, TO cap 204 may be welded to header base 210. Header stem212 may provide mechanical support for chip on sub-mount assembly 214and thermal dissipation. For example, chip on sub-mount assembly 214 maybe attached to header stem 212.

Header base 210 and header stem 212 may be manufactured using acopper-tungsten based material, such as a CuW50; a copper-molybdenumbased material, such as CuMo60; and/or the like. In this way, headerbase 210 and header stem 212 may be CTE matched to reduce a thermaldistortion of header stem 212 and/or to permit compensation of thermaldistortion of header stem 212. In some implementations, chip onsub-mount assembly 214 may be aligned to a centerline of the TO packagewhen attached to header stem 212. The centerline of the TO package is anoutput optical axis of the TO package. Additionally, or alternatively,chip on sub-mount assembly 214 may be aligned to within a thresholdamount offset from the centerline of the TO package, such that at anoperating temperature of the TO package, distortion of header stem 212and thermal expansion or compression of header base 210 cause chip onsub-mount assembly 214 and a laser diode thereof to be aligned to thecenterline of the TO package. Based on a CTE match of header stem 212and header base 210, alignment of chip on sub-mount assembly 214 at areference temperature may be selected such that distortion of headerstem 212 is compensated for by thermal expansion or contraction ofheader base 210. In this way, a CTE matched TO package may be used toensure that the laser diode is aligned to the centerline of the TOpackage during operation of the laser diode, thereby improving couplingefficiency relative to a non-CTE matched TO package.

As indicated above, FIGS. 2A and 2B are provided merely as an example.Other examples are possible and may differ from what was described withregard to FIGS. 2A and 2B.

FIGS. 3A and 3B are diagrams of a finite element analysis 300/300′relating to example implementations described herein. FIGS. 3A and 3Bshow an example of a finite element analysis of a non-CTE matched TOheader and a CTE matched TO header, respectively.

As shown in FIG. 3A, a non-CTE matched TO header includes header base302, header stem 304, and a laser diode and chip on sub-mount assembly306. In this case, header base 302 is manufactured from a steel basedmaterial with a first CTE value and header stem 304 is manufactured froma copper based material with a second CTE value. As a result of adifference between the first CTE value and the second CTE value, at anoperating temperature of laser diode and chip on sub-mount assembly 306,header stem 304 is thermally deformed, as shown, causing the laser diodeand chip on sub-mount assembly 306 to deviate from centerline 308 of thenon-CTE matched TO header. As a result, coupling efficiency is reducedcausing degraded performance for an optical communications transmitteror optical communications transceiver that includes the non-CTE matchedTO header.

As shown in FIG. 3B, a CTE matched TO header includes header base 322, aheader stem 324, and a laser diode and chip on sub-mount assembly 326.In this case, header base 322 is manufactured using a steel basedmaterial and header stem 324 is manufactured using a Copper Tungstenbased material (e.g. CuW50, CuW70) resulting in a CTE match betweenheader base 322 and header stem 324. CuW50 may be selected for headerstem 324 to achieve a threshold CTE match with the steel of header base322, and to achieve a relatively low thermal resistance for the TOheader, which may enable a threshold level of chip and/or packagereliability. In this case, CuW50 may be associated with a 260 W/m-Cthermal conductivity value, thereby achieving the relatively low thermalresistance and permitting the CTE match.

As a result of the CTE match, at an operating temperature, header stem324 is thermally deformed, as shown, by a reduced amount relative toheader stem 304. As a result, laser diode and chip on sub-mount assembly326 deviate from centerline 328 by a reduced amount relative to thedeviation of laser diode and chip on sub-mount assembly 306 fromcenterline 308. Based on the reduced deviation of laser diode and chipon sub-mount assembly 326 from centerline 328, a coupling efficiency isimproved (i.e., a reduction in coupling efficiency is reduced) for anoptical communications transmitter or optical communications transceiverthat includes the CTE matched TO header.

As indicated above, FIGS. 3A and 3B are provided merely as examples.Other examples are possible and may differ from what was described withregard to FIGS. 3A and 3B.

FIG. 4 is a diagram of example results of determining atemperature-induced coupling change (TCC) for a set of header stemmaterials. As shown in FIG. 4, a probability plot 400 is determined fora temperature-induced coupling change at an operating temperature of 75degrees C. relative to a reference temperature or ambient temperature of25 degrees C. As shown by reference number 402, a first header stem ismanufactured from a copper (Cu) based material. As shown by referencenumber 404, a second header stem is manufactured from a copper-tungsten(CuW50) based material. Assume that each header stem is attached to asteel header base. In this case, as shown in probability plot 400, useof copper stem 402 results in a greater temperature-induced couplingchange for a laser diode attached to copper stem 402 relative to a laserdiode attached to CuW50 stem 404. In this way, use of CuW50 or anothersimilarly CTE matched material for a header stem improves opticalperformance of a laser diode mounted to the CTE-matched header stemrelative to a non-CTE matched header stem.

As indicated above, FIG. 4 is provided merely as an example. Otherexamples are possible and may differ from what was described with regardto FIG. 4.

In this way, a CTE matched header stem, such as a copper-tungsten (e.g.,CuW50, CuW60, CuW70, etc.) header stem, in a TO package results inimproved optical performance for a laser diode mounted to the CTEmatched header stem relative to a non-CTE matched header stem withoutreducing thermal dissipation functionality of the header stem.

The foregoing disclosure provides illustration and description, but isnot intended to be exhaustive or to limit the implementations to theprecise form disclosed. Modifications and variations are possible inlight of the above disclosure or may be acquired from practice of theimplementations.

Even though particular combinations of features are recited in theclaims and/or disclosed in the specification, these combinations are notintended to limit the disclosure of possible implementations. In fact,many of these features may be combined in ways not specifically recitedin the claims and/or disclosed in the specification. Although eachdependent claim listed below may directly depend on only one claim, thedisclosure of possible implementations includes each dependent claim incombination with every other claim in the claim set.

No element, act, or instruction used herein should be construed ascritical or essential unless explicitly described as such. Also, as usedherein, the articles “a” and “an” are intended to include one or moreitems, and may be used interchangeably with “one or more.” Furthermore,as used herein, the term “set” is intended to include one or more items(e.g., related items, unrelated items, a combination of related items,and unrelated items, etc.), and may be used interchangeably with “one ormore.” Where only one item is intended, the term “one” or similarlanguage is used. Also, as used herein, the terms “has,” “have,”“having,” or the like are intended to be open-ended terms. Further, thephrase “based on” is intended to mean “based, at least in part, on”unless explicitly stated otherwise.

What is claimed is:
 1. A transistor outline (TO) header for coupling toa TO cap, comprising: a copper-tungsten header stem with a firstcoefficient of thermal expansion (CTE) value; and a header base with asecond CTE value that is different than the first CTE value, the headerbase being a different material from the copper-tungsten header stem. 2.The TO header of claim 1, where the TO header and the TO cap form a TOpackage.
 3. The TO header of claim 1, where the first CTE value iswithin a threshold percentage of the second CTE value.
 4. The TO headerof claim 1, where the header base is a steel header base.
 5. The TOheader of claim 1, where the copper-tungsten header stem is aligned withan output optical axis of the TO header at an operating temperature ofthe TO header.
 6. The TO header of claim 5, where the copper-tungstenheader stem is aligned with the output optical axis within a thresholdamount.
 7. The TO header of claim 5, where the copper-tungsten headerstem is to receive a chip on sub-mount assembly such that a laser diodeis aligned to the output optical axis of the TO header.
 8. An opticalmodule, comprising: a laser diode; a transistor outline (TO) cap; and aTO header, the TO header comprising: a header base to attach to the TOcap, and a copper-tungsten header stem to attach to the header base andto mount the laser diode.
 9. The optical module of claim 8, where thecopper-tungsten header stem is offset from a centerline of the opticalmodule at a reference temperature such that, at an operating temperatureof the optical module, the copper-tungsten header stem is aligned to thecenterline of the optical module.
 10. The optical module of claim 9,where the operating temperature is between approximately 0 degreesCelsius and approximately 75 degrees Celsius.
 11. The optical module ofclaim 9, where the reference temperature is approximately 25 degreesCelsius.
 12. The optical module of claim 8, where the TO cap is weldedto the header base.
 13. The optical module of claim 8, where the headerbase is steel.
 14. The optical module of claim 8, where thecopper-tungsten header stem includes at least one of: a CuW50 material,a CuW60 material, or a CuW70 material.
 15. A transistor outline (TO)package, comprising: a TO cap; and a TO header comprising: a header stemof a first material and a first coefficient of thermal expansion (CTE)value, and a header base of a second material and a second CTE value,the first material and the second material being different, the firstCTE value and the second CTE value being different, the first CTE valueand the second CTE value being within a threshold percentage of eachother.
 16. The TO package of claim 15, where the threshold percentage isapproximately 20%.
 17. The TO package of claim 15, where the firstmaterial includes at least one of: a copper-tungsten based material, ora copper-molybdenum based material.
 18. The TO package of claim 15,where the header stem is to receive an optical component.
 19. The TOpackage of claim 18, where the optical component is a laser diode. 20.The TO package of claim 18, where the TO cap includes a lens tooptically couple the optical component to an optical fiber.